Bifurcated electroacoustic delay lines

ABSTRACT

A bifurcated electroacoustic delay line exhibits at least a pair of substantially identical branches, each assuming a generally pentagonal profile. Each branch is configured to provide (2n+3) reflections of a desired wave at five reflective facets. The reflective pattern so defined tends to maximize the surface area available for the deposition of absorbent material so as to thereby and otherwise minimize spurious transmissions. 
     The branches are physically joined along at least one side and include transducers for the conversion of electrical energy to acoustical energy. An inductance connected between two such transducers couples energy from one branch to the other.

TECHNICAL FIELD

This invention relates to electroacoustic delay lines and moreparticularly, to a delay line characterized by a bifurcated transmissionmedium, comprising at least two branches exhibiting substantiallyidentical profiles, the branches being physically joined alongrespective common sides, said sides remote with respect to other sideswhich are adapted for the attachment of transducers.

BACKGROUND OF THE INVENTION

Electroacoustic delay lines, especially those operating at ultrasonicfrequencies, have found widespread use in various types of electronicequipment; exemplary equipment include radar, computers, color televisonreceivers and video recorders. A primary function of the delay line isthe storage, or delayed transmission, of either analog or digitalinformation for time durations in the microsecond to millisecond range.

Delay lines fabricated from solid transmission media, such as quartz,have assumed a wide variety of shapes and sizes, perhaps the simplestbeing a substantially straight rod provided with a piezoelectrictransducer at each end. The transducers serve to convert electricalsignals to ultrasonic waves at the input and ultrasonic waves toelectrical signals at the output. See FIG. 1.

Because the time delay from input to output depends, of course, on theeffective distance the ultrasonic wave is required to travel, thephysical size of the delay line tends to be proportionate to the timedelay required. A delay line such as is shown in FIG. 1 would require arod approaching an impracticable length in order to effect relativelylong delays, as are often required. Be aware that typical transmissionmedia require roughly a 2.5 mm effective signal path in order to providea one microsecond delay. This drawback has impelled the development ofdelay line configurations more sophisticated than that of the simple rodshown in FIG. 1.

One approach to a reduction in delay line size is characterized by atransmission medium shaped as a regular or an irregular polygon. In sucha configuration the signal path is "folded" through the use of multiplereflections at the facets of the polygon. U.S. Pat. Nos. 2,672,590,entitled "Delay Line", and 2,839,731, entitled "Multifacet UltrasonicDelay Line", both by McSkimer et al., exemplify these delay lineconfigurations.

Even longer delays may be effectuated by "double-deck" delay lines suchas shown in FIGS. 2A and 2B. The double-deck delay lines in factcomprise two substantially identical delay lines 21 and 22 in which theultrasonic wave is coupled from one to the other through the operationof a V-shaped coupling wedge 23.

In the polygonally-configured delay lines alluded to above, the numberof wave reflections and, consequently, the total delay provided, arelimited by the following factors:

(1) As the number of polygonal facets is increased, the tolerance towhich the transmission medium is to be ground becomes more stringent inorder to circumvent the cumulative effect of deviations from the nominalangles of incidence and reflections at each of the facets.

(2) As the number of reflection is increased, the supression of"spurious" signals becomes more problematic. The spurious signals may beassumed to be at least four-fold in origin:

(i) Signals caused by impedance mismatch at the input or outputtransducer and which result in reflections at those transducers. Thesesignals will be present at the output at integer multiples of thedesired delay time.

(ii) Signals caused by insufficient directivity of the transducer. Towit: signals that travel paths other than the desired signal path andtherefore appear at the output displaced by random time intervals fromthe desired signal.

(iii) Signals derived from reflections of the desired signal atirregularities such as cracks and surface roughness in the solid medium.

(iv) Signals directly coupled from the input to output transducer.

(3) Reduction in the physical size of the transducer is attended by areduction of the size of the transducers, and a commensurate degradationin the transducer directivity. Degraded transducer directivitycontributes to the occurrence of spurious signals alluded to above.

(4) As the number of reflections is increased, the cumulative scatteringof the desired signal because of surface roughness or otherirregularities at each reflection is increased.

(5) As the number of reflections is increased, the total volume of thetransmission medium devoted to the desired signal path is increased. Ina typical solid transmission delay line unused portions of the medium(that is, those portions not required for the propogation of the desiredsignal) are coated with a damping material in order to suppress spurioussignals. Because the area allowed to be coated is diminished, theabsorbent effect on spurious signals is likewise diminished.

For example, FIG. 3 depicts a delay line as it might be used in a colortelevision receiver in order to insert a 63.943±0.005 microsecond delay.(This delay, equal to approximately the horizontal line period, is oftenrequired as a part of video enhancement techniques such as combfiltering.) The delay line in FIG. 3 is characterized by a total ofeight reflections occurring at four facets. The blackened areas on thesurface of the delay line represent areas available for the depositionof a damping material. In a specific embodiment, each of these areasmight be approximately 2 millimeters square.

By way of comparison, FIG. 4 depicts a delay line of equivalent durationbut characterized by twelve reflections occurring at the four facets.Each of the blackened areas is approximately 0.6 millimeter square,respresenting a surface area less than one-tenth that of the areas inFIG. 3. Significantly, the spurious response level of the unit in FIG. 3was measured at -40 db with respect to the desired signal while thecorresponding measurement performed on the unit of FIG. 4 resulted in afigure of -25 db.

Accordingly this invention is directed to a delay line that achieves thedesired delay duration in a physically small device through theincorporation of a low number of reflections. The transmission mediumshould exhibit the desired suppression of unwanted reflections.Furthermore, it is desired that a device with the above attributes besusceptible of large volume, economical production.

DISCLOSURE OF THE INVENTION

The above and other objects, advantages and capabilities are acheived inone aspect of the invention by a bifurcated electroacoustic delay linethat includes at least two branches assuming substantially identicalprofiles. In a specific embodiment, the profiles are pentagonal and arecharacterized by a first side, by second and third sides extendingsubstantially perpendicular from the first side in a substantiallyparallel relationship with each other, by a fourth side extending fromthe second side so as to form a first obtuse angle, and by a fifth sideextending from the third side so as to form a second obtuse anglesubstantially equivalent to the first. The fourth and fifth sidesintersect along a line at which they form a third obtuse angle. The twobranches are physically joined along their respective fourth and fifthsides to form an area of bifurcation.

The delay line also includes a set of transducers, an input transducerand an output transducer associated with each of the branches. Thetransducers are arranged so that the input transducer of the firstbranch is positioned laterally adjacent the output transducer of thesecond branch and the output transducer of the first branch ispositioned laterally adjacent the input transducer of the second branch.The output transducer of the first branch is electrically coupled by apassive reactive element to the input transducer of the second branch.

The electroacoustic delay line as disclosed herein permits theeffectuation of specific time delay by virtue of a delay line tending tominimal size. Because the profiles of the branches are identical, thedelay line is easily fabricated through the use of a sawblade configuredso as to bifurcate the transmission medium as it slices individual unitsfrom a blank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a relatively straight-forward implementation of anelectroacoustical delay line.

FIGS. 2A and 2B depict a "double-deck" delay line providing longerdelays at a reduced delay line size.

FIG. 3 depicts a delay line used in color television receivers andillustrates areas on the surface of the delay line available fordeposition of absorbent material.

FIG. 4 depicts a delay line similar to that of FIG. 3 except providingan increased number of reflections at the sides of a delay line ofcomparable surface area.

FIGS. 5A, 5B and 5C depict a delay line exhibiting a substantiallypentagonal profile in accordance with the subject invention.

FIG. 6 depicts a bifurcated delay line having two branches oftransmission medium joined along sides remote from the transducers.

FIG. 7A depicts a bifurcated delay line having two branches of thetransmission medium joined at a side immediate the transducers.

FIG. 7B depicts a bifurcated delay line with diagonal coupling betweenthe two branches.

DESCRIPTION OF A PREFERRED EMBODIMENT

For a better understanding of the subject invention, together with theobjects, advantages and capabilities thereof, reference is made to thefollowing disclosure and appended claims in junction with the abovedescription of some of the aspects of the invention.

Referring now to FIG. 5A, the subject delay line is characterized by afirst side 1 upon which are bonded transducers 11 and 12. Extendingperpendicularly to side 1 and parallel to each other are sides 2 and 3.The parallel extending sides are characterized by substantially equallengths. Sides 2 and 3 terminate at sides 4 and 5 to form an angle ofapproximately 135 degrees at the point of their intersection.

Referring now to FIG. 5B, it can be seen that an electrical signalapplied to input transducer 11, subsequent its conversion to anultrasonic acoustic wave by that transducer, propogates along a path,indicated by the broken line, that includes five reflections occurringrespectively in order at sides 4, 3, 1, 2 and 5. The wave terminates atoutput transducer 12 where it is re-converted from acoustical toelectrical energy. The total length of the path traveled by the desiredacoustic wave can be shown to be, in units of "A" (see FIG. 5A),:respectively, side 4 forming an obtuse angle of approximately 112.5degrees with side 2, side 5 forming a substantially equivalent anglewith side 3. Sides 4 and 5 form an angle of approximately 135 degrees atthe point of their intersection.

L=2/3 (3K-7)(1+square root 2) and that the width, W, and height, H, ofthe transducer, also shown in FIG. 5A, are given by the relationships:

W=2/3 (3K-15+2 square root 2) and

H=2, where K is the number of reflections. Five reflections are shown inthe embodiment of FIG. 5. K can be generalized to take on values to2n+3, where n=1, 2, 3, etc.

As can be seen from FIGS. 5A and 5B, the path traveled by the acousticsignal can be broken down into segments 21, 22, 23, 24, 25 and 26.Segment 21 defining the portion between the input transducer 11 and thefirst reflection, segment 22 between the first and second reflections,segment 23 between the second and third reflections, segment 24 betweenthe third and fourth reflections, segment 25 between the fourth andfifth reflections, and segment 26 between the fifth reflection and theoutput transducer.

A salient advantage of the configuration defined above is that itprovides large areas of the transmission medium unoccupied by thedesired signal. These areas 31, 32, 33, 34, 35 and 36 in FIG. 5B areavailable for efficient screening of the delay line surface withabsorbing material as alluded to above. As can be seen in FIG. 5B, theseareas are circumscribed by the various path segments and the sidesbounding the transmission medium. In particular, area 31 is bounded bysegments 21, 25, 22, 26, 23 and 24; area 32 is bounded by segments 22and 25 and by sides 4 and 5; area 33 is bounded by segments 21 and 24and by sides 1 and 2; area 34 is bounded by segments 21 and 25 and bysides 2 and 4; area 35 is bounded by segments 26 and 23 and by sides 1and 3; area 36 is bounded by segments 26 and 22 and by sides 5 and 3. Inaddition, since only a small part of sides 2 and 3 are occupied by thereflecting ultrasonic wave, the remaining area can be used for clampingthe slab in a housing without deleterious effects on the performance ofthe delay line.

FIGS. 6, 7A, and 7B illustrate bifurcated delay line configurationsincorporating the innovations described above in order to obtain delaysof still longer duration.

Each of these configurations comprise two substantially identicalbranches those branches themselves are substantially identical to thedelay lines shown in FIGS. 5A and 5B.

The delay line of FIG. 6 includes two branches physically joined atsides 4 and 5, the sides remote from the transducers. The delay lineincludes an input transducer T1, an output transducer T2 andintermediate transducers T3 and T4. T3 and T4 are coupled by anelectrical coupling element in the form of a variable inductance, seriescoil L1.

In operation the electrical input signal is applied to input tranducerT1, converted to an acoustic wave that propogates a path confined to thefirst branch (as described above) whereby it eventually arrives attransducer T3. At T3 it is re-converted to electrical energy. Thisenergy is electrically coupled via L1 to transducer T4, and againconverted to acoustic energy so that it propogates the length of thesecond branch and back, according to the path defined by the reflectivepattern detailed above. At T2 the acoustical energy is again convertedto electrical energy.

The delay lines shown in FIGS. 7A and 7B are similar to that of FIG. 6except that the two segments are joined at side 1, the side immediate toand at which the four transducers are bonded. In general it, ispresently believed that the configurations shown in FIGS. 7A and 7B arein some respects preferable to that shown in FIG. 6. In particular,beamspreading (or divergence) of the mechanical wave in the neighborhoodof the transducers can be expected to cause energy to propogate directlybetween T1 and T2 and between T3 and T4. The beamspreading phenomenawould manifest itself as a degradation in the delay line suppression ofspurious responses. In the embodiment of FIG. 6, the bifurcated delayline branches perform a "waveguide" function so as to confine thedesired wave and prevent spreading into laterally adjacent branches ortransducers.

It is expected that the beamspreading effect may be ameliorated bydiagonally coupling L1 from T2 to T3 as shown in FIG. 7B. In this way T4becomes the "output" transducer so that the effect of divergence from T1to T2 is mitigated. That is, the input wave will no longer directlydiverge to the output transducer so as to circumvent the time delayinterposed by progogation through the delay line.

The bifurcated delay lines are characterized by a number of significantadvantages. To wit: The bifurcated unit can be produced at substantiallythe same cost as a single delay line and the bonding of the transducersto the solid medium can be done in the single operation required for asingle delay line. Splitting of the delay line into two branches orsections can be accommodated by a sawblade that bifurcates the delayline as it slices the separate delay lines from the blank. Thisoperation is inherently more efficient than one which requiresindividual slicing of the branches and subsequent physical and/orelectrical re-connection to form a bifurcated structure.

Because the delay line units are cut from the same crystal, impedancematching between the segments is inherently precise. In addition, theseries coupling coil L1, can be used to tune the unit's center frequencyand its adjustment allows a small adjustment in the delay line'scomposite frequency response. Various techniques may be utilized inorder to tune L1. For example, L1 may be wound on a magnetic core andthe position of the core varied relative to the inductive winding so asto vary the effective inductance presented by L1. More simply, thewindings of L1 maybe separated or compressed so as to vary itsinductance.

Furthermore, a given delay time can be implemented with a delay linehaving approximately half the height and/or width of a standard lineexhibiting a similar reflective pattern. Although the thickness of theunit will approach two to three times the thickness of the standard(single-section) unit, this is not deemed a significant drawbackinasmuch as the thickness dimension is substantially less significantthan other dimensions in that it presents a lesser impediment tominiaturization of the delay line.

It should be noted that the number of energy conversions (at thetransducers) is twice the number of conversions that occur in standard,single-branch, delay lines so that the delay line insertion loss tendsto be greater than (but not necessarily twice) that of standard units.Nevertheless, the increase in insertion loss has been found to be lessthan 5 db for a 63.943 microsecond delay line used in color televisionapplication.

Accordingly, while there have been disclosed and described what atpresent are considered to be the preferred embodiments of ultrasonicdelay lines, it will be obvious to those having ordinary skill in theart that various modifications may be made therein without departurefrom the scope of this invention as defined by the appended claims.

INDUSTRIAL APPLICABILITY

The subject invention is useful in all electronic equipment utilizingdelay lines, especially equipment in which minimal delay line size isdesirable.

What is claimed is:
 1. A bifurcated electroacoustic delay linecomprising: a first branch of transmission medium assuming asubstantially pentagonal profile, said profile characterized by:(a) afirst side, (b) second and third sides extending substantiallyperpendicularly from the first side and in a substantially parallelrelationship with each other, (c) a fourth side extending from thesecond side so as to form a first obtuse angle with the second side, and(d) a fifth side extending from the third side so as to form a secondobtuse angle with the third side, so that said fourth and fifth sidesform a third obtuse angle at a point of intersection; said delay linefurther comprising: a second branch of transmission medium assuming aprofile substantially identical to the profile assumed by the firstbranch of transmission medium, wherein said first and second branchesare physically joined along their respective fourth and fifth sides, afirst input transducer bonded to the first side of the first branch, afirst output transducer bonded to the first side of the first branch, asecond input transducer bonded to the first side of the second branch, asecond output transducer bonded to the first side of the second branch,wherein the transducers are arranged so that the first input transducerand second output transducer are positioned laterally adjacent eachother and the first output transducer and second input transducer arepositioned laterally adjacent each other, and means for electricallycoupling the first output and second input transducers.
 2. A bifurcatedelectroacoustic delay line as defined in claim 1 wherein said meanscomprises an inductance.
 3. A bifurcated electroacoustic delay line asdefined in claim 2 wherein said inductance comprises means for adjustingthe frequency response of the delay line.
 4. A bifurcatedelectroacoustic delay line as defined in claim 3 wherein said first andsecond obtuse angles are substantially equal.
 5. A bifurcatedelectroacoustic delay line as defined in claim 4 wherein the thirdobtuse angle is substantially equal to 135 degrees.
 6. Anelectroacoustic delay line as defined in claim 5 wherein the inputtransducer, the output transducer and the five sides of each of thebranches are so arranged that a desired wave launched at the inputtranducer travels a path, from the input transducer to the outputtransducer, comprising at least five reflections, the reflectionsoccurring at the sides of the respective branches so that the pathtraveled by a desired wave comprises at least six segments.
 7. Anelectroacoustic delay line as defined in claim 6 wherein the pathsegments circumscribe areas on a surface of the respective branches,said areas available for the deposition of absorbent material useful inthe attenuation of spurious waves.
 8. An electroacoustic delay line asdefined in claim 7 wherein the transmission medium is fabricated fromglass, metal, crystaline or ceramic material.
 9. An electroacousticdelay line as defined in claim 7 wherein the transducers are fabricatedfrom a piezoelectric material.
 10. An electroacoustic delay line asdefined in claim 3 wherein the input transducer, the output transducer,and the five sides are so arranged that a desired wave travels a path,from the input to the output transducer, comprising five reflections,the reflections occurring at sides of the transmission medium in thatthe path traveled by a desired wave consists of six segments.
 11. Anelectroacoustic delay line as defined in claim 10 wherein the pathsegments circumscribe areas on a surface of the transmission medium,said areas available for the deposition of absorbent material useful inthe attenuation of spurious waves.
 12. An electroacoustic delay line asdefined in claim 11 wherein the transmission medium is fabricated fromglass, metal, crystaline or ceramic material.
 13. An electroacousticdelay line as defined in claim 11 wherein the transducer are fabricatedfrom a piezoelectric material.
 14. A bifurcated electroacoustic delayline as defined in claim 1 wherein the input transducer, the outputtransducer, and the five sides of each of the branches are so arrangedthat a desired wave launched at an input transducer propagates along apath, from the input transducer to the respective output transducer,comprising at least five reflections, the reflections occurring at thesides of the respective branches so that the path along which a desiredwave propagates comprises at least six segments.
 15. A bifurcatedelectroacoustic delay line as defined in claim 14 wherein the pathsegments circumscribe areas on a surface of the respective branches,said areas available for the deposition of absorbent material for theattenuation of spurious waves.
 16. A bifurcated electroacoustic delayline as defined in claim 15 wherein said means comprises and inductance.17. A bifurcated electroacoustic delay line as defined in claim 16wherein said inductance includes a mechanism for adjusting the frequencyresponse of the delay line.
 18. A bifurcated, solid transmission mediumfor an electroacoustic delay line, the medium comprising twosubstantially identical branches, each branch assuming a generallypentagonal profile, the pentagonal profile characterized by a firstside, second and third sides extending substantially perpendicularlyfrom the first side in a substantially parallel relationship withrespect to each other, a fourth side extending from the second side soas to form a first obtuse angle with the second side, and a fifth sideextending from the third side so as to form a second obtuse anglesubstantially equal to the first obtuse angle, so that the fourth andfifth sides form a third obtuse angle at a point of intersection,wherein said branches are joined along their respective fourth and fifthsides.
 19. A bifurcated, solid transmission medium for anelectroacoustic delay line as defined in claim 18 wherein the thirdobtuse angle is substantially equal to 135 degrees.
 20. A bifurcated,solid transmission medium for an electroacoustic delay line as definedin claim 19 wherein each of the branches is constructed so as to providea path of propogation for waves originating at the first side of thebranch, said path comprising:(1) a first segment extending substantiallyparallel to the second side and between the first side and (2) a firstreflection occurring at the fourth side, (3) a second segment extendingbetween the first reflection and (4) a second reflection occurring atthe third side, (5) a third segment extending between the secondreflection and (6) a third reflection occurring at the first side, (7) afourth segment extending between the third reflection and (8) a fourthreflection occurring at the second side, and (9) a fifth segmentextending between the fourth reflection, and (10) a fifth reflectionoccurring at the fifth side, and (11) a sixth segment extendingsubstantially parallel to the third side and between the fifthreflection and the first side.
 21. A solid transmission medium asdefined in claim 20 wherein the path segments circumscribe areas on therespective surfaces of the branches, said areas available for thedeposition of absorbent material for the attenuation of spurious waves.22. A solid transmission medium as defined in claim 21 and fabricatedfrom glass, metal, crystaline or ceramic material.